Secondary batteries refer to devices which convert external electrical energy into chemical energy, store the electrical energy and generate electricity from the chemical energy when necessary. Secondary batteries are also called “rechargeable batteries,” which means that they are capable of repeated charge and discharge. Lead-acid batteries, nickel cadmium (NiCd) batteries, nickel metal hydride (NiMH) batteries, lithium ion batteries and lithium ion polymer batteries are frequently used as secondary batteries. Secondary batteries offer economic and environmental advantages over primary batteries that are disposed of after energy stored therein has been exhausted.
Secondary batteries are currently used in applications where low power is needed, for example, devices for assisting in the start-up of car engines, portable devices, instruments and uninterrupted power supply systems. Recent developments in wireless communication technologies have led to the popularization of portable devices and have brought about a tendency to connect many kinds of existing devices to wireless networks. As a result, demand for secondary batteries is dramatically increasing. Secondary batteries are also used in environmentally friendly next-generation vehicles such as hybrid vehicles and electric vehicles to reduce the costs and weight and to increase the service life of the vehicles.
Generally, most secondary batteries have a cylindrical, prismatic or pouch type shape depending on the fabrication process thereof. That is, a secondary battery is typically fabricated by inserting an electrode assembly composed of an anode, a cathode and a separator into a cylindrical or prismatic metal can or a pouch-type case made of an aluminum laminate sheet, and injecting an electrolyte into the electrode assembly. Accordingly, the cylindrical, prismatic or pouch-type secondary battery requires a certain space for assembly, which is an obstacle to the development of various types of portable devices. Thus, there is a need for a novel type of secondary battery that is easily adaptable in shape. In response to this need, highly flexible linear batteries, for example, cable-type secondary batteries with a high ratio of length to cross-sectional diameter have been proposed.
However, such a cable-type secondary battery requiring high flexibility is prone to deformation because it is exposed to frequent external physical impact in view of its structural characteristics. For example, the cable-type secondary battery may be bent by frequent external physical impact. This deformation increases the risk of disconnection during use. Another problem is that an anode active material, such as Si or Sn, is separated when electrodes of the cable-type secondary battery are expanded and contracted during repeated charge and discharge. In this case, the cable-type secondary battery suffers from a more serious deterioration in performance by frequent physical impact than general secondary batteries do.